A. Field of the Invention
This invention relates to thermosetting polymerizable compositions and particularly to flame-retardant thermosetting molding compositions.
B. Description of the Prior Art
Thermosetting polymerizable compositions, commonly termed thermosetting resins, are widely used in making castings, laminates, and molded articles. The high strength to weight ratios of thermoset resins when properly reinforced and cured and the resistance of these compositions to environmental conditions have suggested the application of thermoset compositions in numerous applications, including many applications where combustibility is a serious consideration.
A number of methods have been employed to reduce the rate of combustion of thermoset resin compositions. These methods have included the use of numerous flame-retardant additives, including inorganic additives, organic additives, a combination of inorganic and organic additives, the addition of a flame-retardant element that reacts and becomes part of the thermoset resin, and combinations of all these methods.
The use of inorganic fillers to impart flame retardance is attractive on a cost basis. Typical inert flame-retardant fillers retard the burning rate to the extent that they act as diluents of the combustible components. However, large amounts of fillers adversely affect the electrical properties of thermoset resin compositions.
Organic compounds containing halogen and phosphorous atoms such as chlorinated paraffins, chlorinated biphenyls, chlorinated aryl substituted alkyls, phosphorus containing hydrocarbons, and the like have been used for many years to impart flame resistance to plastic compositions including thermoset resins. The chlorinated materials are quite effective as flame retardants. The flame-retarding mechanism is believed to involve the release of hydrogen chloride at combustion temperatures which snuff out the flame by exclusion of oxygen. Only highly chlorinated organic materials have been considered, since the objective is to provide maximum flame resistance with a minimum of additive material. These highly chlorinated organics, including those which are solid, have a plasticizing action on thermoset compositions. Generally the plasticizing action of the chlorinated organic compounds lowers the heat deflection temperature and otherwise impairs the desirable properties of cured thermosetting resins.
Combinations of chlorinated organic additive with antimony trioxide have been known for a number of years. The effect of certain other metal oxides, such as magnesia or hydrated alumina, combined with antimony oxides in flameproofing chlorinated polymers, was noted by D. C. Thompson et al in an article entitled "Flame Resistance of Neoprene" appearing in the August 1958 issue of Rubber Age. Combinations of hydrated alumina with antimony oxide and chlorinated organics are widely recognized as effective flame retardants for resinous compositions including thermoset resin compositions.
Satisfactory flame resistance in thermoset polymers has been obtained by using reactive chlorinated intermediates which become a part of the cured thermoset system, thus avoiding the plasticizing effect experienced when using chlorinated paraffins, chlorinated polyphenyls and the like. An important advance in the chemistry of self-extinguishing or flame-retardant thermoset resins occurred as a result of the discovery of a commercial method of making chlorendic acid or anhydride in 1952. Chlorendic acid and anhydride have been esterified to form polyesters similar to those obtained when using phthalic anhydride. Additionally, chlorendic acid and anhydride have been used to prepare reactive monomers, such as diallyl chlorendate, which are useful in flameproofing thermosetting resins such as polyesters and diallylic phthalates without substantially reducing the desirable physical properties of the thermoset resin compositions.
The addition of a reactive flame-retardant additive to thermoset resins such as polyesters and diallylic phthalate resins is highly desirable, as this method of flameproofing offers flexibility in resin production, since it is possible to flameproof general-purpose resins rather than to make special flame-retardant grade resins. The chlorendic acid-based reactive flame-retardant materials are expensive and since 1952 no efficient and inexpensive reactive flame retardants for thermoset resins have been widely accepted. The principal object of this invention is the preparation of highly flame-retardant, inexpensive, thermosetting resin compositions.